Emulsions, in general, are heterogeneous systems of one immiscible liquid dispersed in another in the form of droplets which usually have a diameter greater than 1 micron. The two liquids are chemically unreactive and form systems characterized by a low thermodynamic stability. Simple emulsions are classified according to the nature of their continuous and dispersed phase. It is the custom to set forth the droplet (dispersed) phase first followed by the continuous phase separated by a / mark, i.e., either water(droplets)-in-oil(continuous) (W/O) or oil-in-water (O/W) emulsions. Multiple emulsions are characterized by listing the primary emulsion set first which is dispersed in a continuous phase. For example, in a water-in-oil-in-water (W/O/W) multiple emulsion, a W/O primary emulsion is dispersed in a water continuous phase. An emulsifier is present to stabilize the emulsion and a variety of ionic and non-ionic surfactants are available for this purpose. Lipophilic (oil-soluble, low HLB) surfactants are used to stabilize W/O emulsions, whereas hydrophilic (water-soluble, high HLB) surfactants are used to stabilize oil/water systems.
Multiple emulsions are more complex systems as the drops of the dispersed phase themselves contain even smaller dispersed droplets which normally consist of a liquid which is miscible, and in most cases, is identical with the continuous phase. They are therefore, emulsions of emulsions. For each type of multiple emulsion, the internal and external phases are alike and an intermediate phase separates the two like phases. The intermediate phase is immiscible with the two like phases.
Multiple emulsions are usually prepared by a two-stage procedure; Matsumoto, et al., J. Colloid Interference Sci., 57-353-361 (1976). The first stage involves the preparation of a primary emulsion, which, in the preparation of a W/O/W emulsion, is a W/O emulsion. In the second step, the primary emulsion is further emulsified in water containing a hydrophilic emulsifier to form the multiple emulsion. The primary emulsion may be prepared in any suitable manner; for example, with a laboratory mixer, by ultrasonification, etc. A lipophilic surfactant is used to promote the formation of a W/O emulsion. This emulsion is then poured into a solution or a dispersion of a hydrophilic surfactant in water. The hydrophilic surfactant is used to promote O/W emulsification in which the "oil" phase is the W/O emulsion.
The second emulsification step is critical and sometimes extremely difficult to effect as excess mixing can fracture the drops of the primary emulsion, resulting in a simple O/W emulsion. The internal water droplets are lost and mixed with the external aqueous phase as the oil drops are torn apart. For this reason, high shear mixers and sonification are unsuitable methods for preparation of the second emulsion. A small, low-shear mixing may be employed or the mixture may be shaken by hand. However, no matter what emulsification method is used for the second step, some of the internal aqueous phase is unavoidably lost to the external aqueous phase.
With respect to food technology, considerable research effort has been expended on developing reduced fat food products which have low oil content, particularly aqueous based products which have low or substantially no fat content. Significant advances have been made in reducing fat and oil content of various food products through the use of, for example, water-in-oil emulsions or water-in-oil-in-water emulsions, wherein water occupies volume which otherwise would have been occupied with oil, thereby commensurately reducing the amount of oil in an oil-containing food product. For example, Takahashi, et al., U.S. Pat. Nos. 4,632,840, 4,626,443 and 4,626,444 disclose reduced fat salad dressing having a W/O/W emulsion base. Such salad dressings nevertheless still have about 30% oil by weight. Further fat reductions have been obtained using, as fat mimetics, novel carbohydrate-protein complexes such as those disclosed in U.S. Pat. No. 5,104,674 or microreticulated microcrystalline cellulose as disclosed in co-owned U. S. Pat. No. 5,011,701, filed Aug. 18, 1989. Such carbohydrate-protein complexes or microreticulated microcrystalline cellulose are particularly useful in providing no-fat food products such as viscous and pourable salad dressings and the like having fat-like organoleptic characteristics.
While elimination or substantial reduction of oil content is attainable, such low-fat or no-fat products characteristically lack (or lose during storage) the desirable flavor possessed by their high-fat counterparts. Stabilization of aqueous-soluble or lipid-soluble flavors in low- or no-fat, aqueous based food products has not received much attention. Heretofore, aqueous soluble flavors have merely been added along with other aqueous soluble ingredients to produce low- or no-fat food products; with respect to reintroducing fat-soluble flavors to reduced-fat products. PCT International Application No. W090/00354 to Singer discloses adding to low-fat and no-fat foods, fat globules containing concentrated fat soluble flavoring to simulate the organoleptic effect of fat-rich food products. In each of these cases, the flavors are in contact with the aqueous based food vehicle environment (either directly or at the interface between the fat globules and the aqueous base of the food vehicle) and thereby may be adversely affected. Flavor perception in low- or no-fat food products containing soluble flavors simply mixed into the aqueous-based food vehicle, e.g., viscous or pourable salad dressings or the like, rapidly deteriorates; presumably due to interaction of flavors with the aqueous base, giving such products a short shelf life. It would therefore be desirable to provide aqueous based food products (especially no-fat products) which have aqueous or oil soluble flavor components stably maintained so as to protect the flavors from volatilization, oxidation and other desirable events, during extended storage, while at the same time providing for ready release of such flavors, with good organoleptic characteristics, when such low/no-fat food products are eaten.
In view of the above, it is an object of the present invention to provide stable W/O/W multiple emulsions which utilize reduced amounts of emulsifiers, which multiple emulsions are suitable for food emulsion technology, as well as other applications utilizing multiple emulsions. It is also an object of the present invention to provide flavor delivering compositions which are stably maintained in low-fat or no-fat products, such that the flavors are protected from the aqueous based food vehicle environment until the food product is eaten, whereupon the flavor is readily released to yield a desirable taste perception and mouthfeel. These and other objects of the invention will become apparent from the following detailed description.